[flang][hlfir] `hlfir.char_extremum` lowering

This patch implements the lowering for the `hlfir.char_extremum` operation.

Discussion for this patch can be found in the draft patch [here](https://reviews.llvm.org/D143326). The reason
for not promoting this draft to a true patch for review was because I needed to separate the op
definition/codegen and lowering into two separate patches, as preferred by @jeanPerier.

Depends on D152474

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D152475